[mapserver-users] jagged edges on images

[Ed McNierney]

Chris -
 
I deal with multi-terabyte data sets under MapServer already, and while there is a price for the resampled images, it does only have to be paid once.  Even an extensive set of overviews (2x, 4x, 8x, 16x, etc.) increases the data storage by about a third (the 2x resample is obviously by far the largest).  The additional processing to produce high-quality resampled images is pretty substantial relative to the rest of what MapServer does.
 
However, if you do want to go that route, the best solution right now would be to have MapServer generate the "native resolution" output image - perhaps of a larger size than the output image if needed - and then write an image post-processor that does something like bicubic resampling in RGB space to the desired output size and then send it along to the client.  High-quality resampling is somewhat compute- and memory-intensive, but if your output map images are modest sizes it might not be a big deal.
 
     - Ed

	-----Original Message----- 
	From: chris_faulkner at anytimenow.com [mailto:chris_faulkner at anytimenow.com] 
	Sent: Fri 6/14/2002 12:00 PM 
	To: Ed McNierney 
	Cc: mapserver-users at lists.gis.umn.edu; chris_faulkner at anytimenow.com 
	Subject: RE: [mapserver-users] jagged edges on images
	
	

	Hi
	
	Sorry for the incorrect terminology.
	
	I wouldn't want to store multiple copies of the data because our image database is measured in terrabytes. The cost of the disk is not necessarily of prime importance (although top-end robust, reliable, raid is not cheap when you buy that sort of quantity). The management issues of all those files would come into play, though. I'd rather wait another fraction of a second for it to do the resampling every time, especially when the image is being served over the web and the slowest part of the process is much longer than that and is out of our control - the download ro the user's device.
	
	
	Chris
	
	> Chris -
	>
	> In the example you sent, it's not downsampling, it's upsampling - the
	> output image is at a higher resolution than the input image.  While
	> there are alternatives, and it would be nice for MapServer to support
	> more resampling options, doing that well really needs to wait for the
	> 24-bit output support to be available.
	>
	> Why do you not want to store multiple copies of the data?  With multiple
	> copies you greatly reduce the amount of resampling that needs to be
	> done, and I'm a big fan of precomputing everything you possibly can.
	>
	>  - Ed
	>
	>
	> -----Original Message-----
	> From: chris_faulkner at anytimenow.com
	> [mailto:chris_faulkner at anytimenow.com]
	> Sent: Friday, June 14, 2002 3:41 AM
	> To: Ed McNierney
	> Cc: chris_faulkner at anytimenow.com; mapserver-users at lists.gis.umn.edu
	> Subject: RE: [mapserver-users] jagged edges on images
	>
	>
	> Hi
	>
	> Yes - when I make sure that the input image resolution is the same as
	> output,  the image looks OK.
	>
	> However, as my last email says, I do want to be able to  change scale
	> without storing multiple copies of the data so it would be nice if the
	> down-sampling operation could support an option of a higher quality
	> alogorithm. IN this case, the output image might need to increase in
	> depth, though - maybe MapServer wouldn't support that.
	>
	> Chris
	>
	>
	> > Chris -
	> >
	> > This looks like an artifact of image scaling in the output image.
	> > MapServer prudently uses a fast algorithm for scaling source input
	> > images to produce the output result.  If you ask for an output image
	> > whose pixel resolution (that is, meters per pixel, for example) is
	> > different than the source image resolution, the source image has to be
	> > scaled.  Your sample image looks like an output image with a pixel
	> > resolution slightly smaller than the input image.  For example, let's
	> > assume your source image is at a resolution of 2 meters per pixel.
	> Then
	> > let's assume you asked for a 400 x 400 pixel output image from
	> MapServer
	> > with a spatial extent of 600 meters by 600 meters.  That means you
	> want
	> > the output pixels to be 1.5 meters per pixel.
	> >
	> > Since the output is "zoomed in" a bit, MapServer is being asked to
	> > generate 400 output pixels (in each dimension) out of 300 input pixels
	>
	> > (600 meters at 2 meters per pixel).  It will do that by duplicating
	> > every third input pixel in the output, so if you had a row of pixels
	> > with these values:
	> >
	> > 0 1 2 3 4 5
	> >
	> > you would get an output image with these values:
	> >
	> > 0 1 2 2 3 4 5 5
	> >
	> > Image then, that you had two single-pixel vertical lines in the source
	>
	> > image.  In the output, the one at pixel position 2 would end up being
	> > two pixels wide, while the identical input line at pixel position 4
	> > would still be only one pixel wide in the output.  This will look
	> > strange.
	> >
	> > In your specific example, you appear to have antialiased text in the
	> > input image.  The intermediate-shaded pixels along the edge of the
	> > text are sometimes replicated, giving the odd effect you see in the
	> > words "Briar Patch".
	> >
	> > For optimum image quality without artifacts, keep the output image
	> > resolution the same as the source resolution, or a multiple of 2x the
	> > source resolution.  If you're going to be doing that often, consider
	> > using GDAL and create overview images (preprocessed subsamples of the
	> > source image) to avoid having to resample the source image over and
	> > over again to serve map requests.
	> >
	> >  - Ed
	> >
	> > Ed McNierney
	> > President and Chief Mapmaker
	> > TopoZone.com / Maps a la carte, Inc.
	> > 73 Princeton Street, Suite 305
	> > North Chelmsford, MA  01863
	> > ed at topozone.com
	> > (978) 251-4242
	> >
	> > -----Original Message-----
	> > From: chris_faulkner at anytimenow.com
	> > [mailto:chris_faulkner at anytimenow.com]
	> > Sent: Wednesday, June 12, 2002 6:14 PM
	> > To: mapserver-users at lists.gis.umn.edu
	> > Subject: [mapserver-users] jagged edges on images
	> >
	> >
	> > Hello again
	> >
	> > I've been looking at some imagery being generated via PHP Mapscript.
	> > On my imagery , jagged effects appear along some of the edges. The
	> > jagged version from MapServer is attached. Look along the road
	> > boundaries and the word "Briar Patch". Anyone know why this happens ?
	> > These effects don't appear when I view and zoom in and out of the
	> > source with ImageMagick. Here is the tiffinfo output. Note that I
	> > started with tiled Tifs and converted to stripped - I have played with
	>
	> > different sized strips.
	> >
	> > TIFF Directory at offset 0x1255d6
	> >   Image Width: 3500 Image Length: 3500
	> >   Bits/Sample: 8
	> >   Compression Scheme: PackBits
	> >   Photometric Interpretation: palette color (RGB from colormap)
	> >   FillOrder: msb-to-lsb
	> >   Document Name: "Standard Input"
	> >   Image Description: "converted PNM file"
	> >   Orientation: row 0 top, col 0 lhs
	> >   Samples/Pixel: 1
	> >   Rows/Strip: 2
	> >   Planar Configuration: single image plane
	> >   Color Map: (present)
	> >
	> > If anyone needs it, I can send the original one but I didn't want to
	> > waste too much bandwidth !
	> >
	> > Thanks
	> >
	> >
	> > Chris
	> >
	> >
	>
	>